Submillimeter Dust Continuum Emission Research Articles

Z-GAL: A NOEMA spectroscopic redshift survey of bright Herschel galaxies

The z-GAL survey observed 137 bright Herschel-selected targets with the IRAM Northern Extended Millimeter Array, with the aim to measure their redshift and study their properties. Several of them have been resolved into multiple sources. Consequently, robust spectroscopic redshifts have been measured for 165 individual galaxies in the range 0.8 < z < 6.5. In this paper we analyse the millimetre spectra of the z-GAL sources, using both their continuum and line emission to derive their physical properties. At least two spectral lines are detected for each source, including transitions of 12CO, [CI], and H2O. The observed 12CO line ratios and spectral line energy distributions of individual sources resemble those of local starbursts. In seven sources the para-H2O (211−202) transition is detected and follows the IR versus H2O luminosity relation of sub-millimetre galaxies. The molecular gas mass of the z-GAL sources is derived from their 12CO, [CI], and sub-millimetre dust continuum emission. The three tracers lead to consistent results, with the dust continuum showing the largest scatter when compared to 12CO. The gas-to-dust mass ratio of these sources was computed by combining the information derived from 12CO and the dust continuum and has a median value of 107, similar to star-forming galaxies of near-solar metallicity. The same combined analysis leads to depletion timescales in the range between 0.1 and 1.0 Gyr, which place the z-GAL sources between the ‘main sequence’ of star formation and the locus of starbursts. Finally, we derived a first estimate of stellar masses – modulo possible gravitational magnification – by inverting known gas scaling relations: the z-GAL sample is confirmed to be mostly composed by starbursts, whereas ∼25% of its members lie on the main sequence of star-forming galaxies (within ±0.5 dex).

STAR FORMATION RELATIONS IN THE MILKY WAY

ABSTRACT The relations between star formation and properties of molecular clouds (MCs) are studied based on a sample of star-forming regions in the Galactic Plane. Sources were selected by having radio recombination lines to provide identification of associated MCs and dense clumps. Radio continuum emission and mid-infrared emission were used to determine star formation rates (SFRs), while 13CO and submillimeter dust continuum emission were used to obtain the masses of molecular and dense gas, respectively. We test whether total molecular gas or dense gas provides the best predictor of SFR. We also test two specific theoretical models, one relying on the molecular mass divided by the free-fall time, the other using the free-fall time divided by the crossing time. Neither is supported by the data. The data are also compared to those from nearby star-forming regions and extragalactic data. The star formation “efficiency,” defined as SFR divided by mass, spreads over a large range when the mass refers to molecular gas; the standard deviation of the log of the efficiency decreases by a factor of three when the mass of relatively dense molecular gas is used rather than the mass of all of the molecular gas.

THE DISTRIBUTION OF MASS SURFACE DENSITIES IN A HIGH-MASS PROTOCLUSTER

ABSTRACT We study the probability distribution function (PDF) of mass surface densities, Σ, of infrared dark cloud (IRDC) G028.37+00.07 and its surrounding giant molecular cloud. This PDF constrains the physical processes, such as turbulence, magnetic fields, and self-gravity, that are expected to be controlling cloud structure and star formation activity. The chosen IRDC is of particular interest since it has almost 100,000 solar masses within a radius of 8 pc, making it one of the most massive, dense molecular structures known and is thus a potential site for the formation of a “super star cluster.” We study Σ in two ways. First, we use a combination of NIR and MIR extinction maps that are able to probe the bulk of the cloud structure up to Σ ∼ 1 g cm−2(A V ≃ 200 mag). Second, we study the FIR and submillimeter dust continuum emission from the cloud utilizing Herschel-PACS and SPIRE images and paying careful attention to the effects of foreground and background contamination. We find that the PDFs from both methods, applied over a ∼20′(30 pc)-sized region that contains ≃1.5 × 105 M ⊙ and enclosing a minimum closed contour with Σ ≃ 0.013 g cm−2 (A V ≃ 3 mag), shows a lognormal shape with the peak measured at Σ ≃ 0.021 g cm−2 (A V ≃ 4.7 mag). There is tentative evidence for the presence of a high-Σ power-law tail that contains from ∼3% to 8% of the mass of the cloud material. We discuss the implications of these results for the physical processes occurring in this cloud.

PLANETARY SIGNATURES IN THE SAO 206462 (HD 135344B) DISK: A SPIRAL ARM PASSING THROUGH VORTEX?

ABSTRACT The disk surrounding SAO 206462, an 8 Myr old Herbig Ae star, has recently been reported to exhibit spiral arms, an asymmetric dust continuum, and a dust-depleted inner cavity. By carrying out two-dimensional, two-fluid hydrodynamic calculations, we find that a planetary-mass companion located at the outer disk could be responsible for these observed structures. In this model, the planet excites primary and secondary arms interior to its orbit. It also carves a gap and generates a local pressure bump at the inner gap edge where a vortex forms through Rossby wave instability. The vortex traps radially drifting dust particles, forming a dust-depleted cavity in the inner disk. We propose that the vortex is responsible for the brightest southwestern peak seen in infrared scattered light and sub-millimeter dust continuum emission. In particular, it is possible that the scattered light is boosted as one of the spiral arms passes through the high density vortex region, although the vortex alone may be able to explain the peak. We suggest that a planetary companion with a mass of 10–15 M J ?> is orbiting SAO 206462 at 100–120 au. Monitoring of the brightest peak over the next few years will help reveal its origin because the spiral arms and vortex will show distinguishable displacement.

A multi-wavelength view of the Galactic center dust ridge reveals little star formation

The Galactic center dust ridge consists of a narrow string of massive condensations identified in submillimeter dust continuum emission. To determine whether new high-mass stars are forming in this region, we performed new observations at 870 $\mu$m with the Atacama Pathfinder Experiment telescope and at 8.4 GHz with the Very Large Array. We complement our data with recent maser and mid-infrared results. The ridge's clouds are dark at mid-infrared wavelengths, indicating the presence of cold, high column density material. In combination with existing temperature measurements in the dust ridge, we determined the masses of the largest clouds. The results show that the dust ridge contains a very massive reservoir of molecular material. We find five radio sources at 8.4 GHz in the general dust ridge vicinity but outside of the dust ridge clouds, which are probably all excited by massive young stars, whose properties we constrain. Our observations exclude the existence of zero age main sequence stars with spectral types earlier than B0.5 within the dust ridge clouds. The only indication of ongoing high-mass star formation inside the clouds are class II methanol masers that are found in two of the clouds. Except for a weak water maser, found in previous observations, no signs of star formation are detected in the massive cloud M0.25+0.012.

13CO and C18OJ = 2–1 mapping of the environment of the Class 0 protostellar core SMM 3 in Orion B9

We attempt to achieve a better understanding of the gas distribution and velocity field around the deeply embedded Class 0 protostar SMM 3 in the Orion B9 star-forming region. Using the APEX 12-m telescope, we mapped the line emission from the J=2-1 rotational transition of two CO isotopologues, 13CO and C18O, over a 4' x 4' region around Orion B9/SMM 3. Both the 13CO and C18O lines exhibit two well separated velocity components at about 1.3 and 8.7 km/s. The emission of both CO isotopologues is more widely distributed than the submillimetre dust continuum emission as probed by LABOCA. The LABOCA 870-micron peak position of SMM 3 is devoid of strong CO isotopologue emission, which is consistent with our earlier detection of strong CO depletion in the source. No signatures of a large-scale outflow were found towards SMM 3. The 13CO and C18O emission seen at ~1.3 km/s is concentrated into a single clump-like feature at the eastern part of the map. The peak H2 column density towards a C18O maximum of the low-velocity component is estimated to be ~10^22 cm-2. A velocity gradient was found across both the 13CO and C18O maps. Interestingly, SMM 3 lies on the border of this velocity gradient. The 13CO and C18O emission at ~1.3 km/s is likely to originate from the "low-velocity part" of Orion B. Our analysis suggests that it contains high density gas, which conforms to our earlier detection of deuterated species at similarly low radial velocities. The sharp velocity gradient in the region might represent a shock front resulting from the feedback from the nearby expanding HII region NGC 2024. The formation of SMM 3, and possibly of the other members of Orion B9, might have been triggered by this feedback.

ATLASGAL: the APEX Telescope Large Area Survey of the Galaxy

Submillimeter dust continuum emission traces high molecular column densities and, thus, dense cloud regions in which new stars are forming. Surveys of the Galactic plane in such emission have the potential of delivering an unbiased view of high-mass star formation throughout the Milky Way. The location of the APEX telescope on the Chajnantor plateau in Chile is ideally suited for mapping the inner Galaxy. ATLASGAL, The APEX Telescope Large Area Survey of the Galaxy at 870 μm, is a survey of the Galactic plane using the Large APEX Bolometer Camera (LABOCA), in the Galactic longitude and latitude ranges of ±60 and ±1.5°, respectively. This survey is providing an unbiased sample of cold dusty clumps in the Galaxy at submillimeter wavelength and a variety of molecular line follow-up observations have been started to characterize the physical and chemical conditions in the newly found clumps. Here, first results from this survey and its follow-up programs are described.

The physical and dynamical structure of Serpens

The Serpens North Cluster is a nearby low mass star forming region which is part of the Gould Belt. It contains a range of young stars thought to correspond to two different bursts of star formation and provides the opportunity to study different stages of cluster formation. This work aims to study the molecular gas in the Serpens North Cluster to probe the origin of the most recent burst of star formation in Serpens. Transitions of the C17O and C18O observed with the IRAM 30m telescope and JCMT are used to study the mass and velocity structure of the region while the physical properties of the gas are derived using LTE and non-LTE analyses of the three lowest transitions of C18O. The molecular emission traces the two centres of star formation which are seen in submillimetre dust continuum emission. In the ~40M_sun NW sub-cluster the gas and dust emission trace the same structures although there is evidence of some depletion of the gas phase C18O. The gas has a very uniform temperature (~10K) and velocity (~8.5km/s) throughout the region. This is in marked contrast to the SE sub-cluster. In this region the dust and the gas trace different features, with the temperature peaking between the submillimetre continuum sources, reaching up to ~14K. The gas in this region has double peaked line profiles which reveal the presence of a second cloud in the line of sight. The submillimetre dust continuum sources predominantly appear located in the interface region between the two clouds. Even though they are at a similar stage of evolution, the two Serpens sub-clusters have very different characteristics. We propose that these differences are linked to the initial trigger of the collapse in the regions and suggest that a cloud-cloud collision could explain the observed properties.

A Large‐Scale Survey of NGC 1333

We observed the clustered star forming complex NGC 1333 with the BIMA and FCRAO telescopes in the transitions HCO+(1-0) and N2H+(1-0) over an 11' × 11' area with resolution ~10'' (0.015 pc). The N2H+ emission follows very closely the submillimeter dust continuum emission, while HCO+ emission appears more spatially extended and also traces outflows. We have identified 93 N2H+ cores using the CLUMPFIND algorithm, and we derive N2H+ core masses between 0.05 and 2.5 M☉, with uncertainties of a factor of a few, dominated by the adopted N2H+ abundance. From a comparison with virial masses, we argue that most of these N2H+ cores are likely to be bound, even at the lowest masses, suggesting that the cores do not trace transient structures, and implies the entire mass distribution consists of objects that can potentially form stars. We find that the mass distribution of N2H+ cores resembles the field star IMF, which suggests that the IMF is locked in at the prestellar stage of evolution. We find that the N2H+ cores associated with stars identified from Spitzer infrared images have a flat mass distribution. This might be because lower mass cores lose a larger fraction of their mass when forming a star. Even in this clustered environment, we find no evidence for ballistic motions of the cores relative to their lower density surroundings traced by isotopic CO emission, although this conclusion must remain tentative until the surroundings are observed at the same high resolution as the N2H+.

Tracing the Mass during Low‐Mass Star Formation. III. Models of the Submillimeter Dust Continuum Emission from Class 0 Protostars

Seven Class 0 sources mapped with SCUBA at 850 and 450 micron are modeled using a one dimensional radiative transfer code. The modeling takes into account heating from an internal protostar, heating from the ISRF, realistic beam effects, and chopping to model the normalized intensity profile and spectral energy distribution. Power law density models, n(r) ~ r^{-p}, fit all of the sources; best fit values are mostly p = 1.8 +/- 0.1, but two sources with aspherical emission contours have lower values (p ~ 1.1). Including all sources, = 1.63 +/- 0.33. Based on studies of the sensitivity of the best-fit p to variations in other input parameters, uncertainties in p for an envelope model are \Delta p = +/- 0.2. If an unresolved source (e.g., a disk) contributes 70% of the flux at the peak, p is lowered in this extreme case and \Delta p = ^{+0.2}_{-0.6}. The models allow a determination of the internal luminosity ( = 4.0 \lsun) of the central protostar as well as a characteristic dust temperature for mass determination ( = 13.8 +/- 2.4 K). We find that heating from the ISRF strongly affects the shape of the dust temperature profile and the normalized intensity profile, but does not contribute strongly to the overall bolometric luminosity of Class 0 sources. There is little evidence for variation in the dust opacity as a function of distance from the central source. The data are well-fitted by dust opacities for coagulated dust grains with ice mantles (Ossenkopf & Henning 1994). The density profile from an inside-out collapse model (Shu 1977) does not fit the data well, unless the infall radius is set so small as to make the density nearly a power-law.

Discovery of a dense bipolar outflow from a new class 0 protostar in NGC 2068/LBS 17

We report the discovery of high-velocity dense gas from a bipolar outflow source near NGC 2068 in the L1630 giant molecular cloud. CO and HCO+J=3→2 line wings have a bipolar distribution in the vicinity of LBS 17-H with the flow orientated roughly east-west and perpendicular to the elongation of the submillimetre dust continuum emission. The flow is compact (total extent ∼0.2 pc) and contains of the order of 0.1 M⊙ of swept-up gas. The high-velocity HCO+ emission is distributed over a somewhat smaller area <0.1 pc in extent. A map of C18O J=2→1 emission traces the LBS 17 core and follows the ambient HCO+ emission reasonably well, with the exception of the direction towards LBS 17-H where there is a significant anticorrelation between the C18O and HCO+. A comparison of beam-matched C18O and dust-derived H2 column densities suggests that CO is depleted by up to a factor of ∼50 at this position if the temperature is as low as 9 K, although the difference is substantially reduced if the temperature is as high as 20 K. Chemical models of collapsing clouds can account for this discrepancy in terms of different rates of depletion on to dust grains for CO and HCO+. LBS 17-H has a previously known water maser coincident with it but there are no known near-infrared, IRAS or radio continuum sources associated with this object, leading to the conclusion that it is probably very young. A greybody fit to the continuum data gives a luminosity of only 1.7 L⊙ and a submillimetre-to-bolometric luminosity ratio of 0.1, comfortably satisfying the criteria for classification as a class 0 protostar candidate.